1. Field of the Invention
The present invention relates to a method and device for fragmenting concretions in human ducts.
2. The Related Art
The technique of using high voltage electrical discharge in a fluid medium to create a shockwave capable of fragmenting rocks was developed in the Soviet Union as an industrial technique over 35 years ago. In the early 1960's, the Soviets successfully applied the principles of electrohydraulic lithotripsy (EHL) in the treatment of bladder calculi. As early as 1973, attempts were made to apply this technique to the treatment of ureteral calculi.
U.S. Pat. No. 4,027,674 and U.S. Pat. No. 4,030,505 are typical of the known art. Therein is described a method and device for catheterization within human ducts so as to remove concretions, plaques and sclerotic clots. A series of high voltage pulses of sufficiently low amperage are generated in this method. These pulses are then dispatched through a flexible conduit containing a positive electrode extending coaxially within the conduit embedded in an electrically insulated material and a negative electrode peripherally embedded in the conduit encircling and coextensive with the positive electrode.
Although the use of EHL in treatment of bladder calculi has proved to be a reliable and essentially safe procedure, treatment of ureteral calculi and biliary calculi with this technique is not widely accepted. There are risks associated with the use of existing EHL electrodes in small body lumens such as the ureter and the biliary tree. Moreover, there are difficulties with manipulation and positioning of the probe electrodes in proper proximity to a target calculus. These miniature probes are also of inadequate reliability and have an undesirably short useful life.
Ultra high speed photographic analysis has been used to document the phenomena that actually occurs during the electrohydraulic lithotripsy of a target calculus. When the high voltage discharge occurs across the two electrodes at the tip of the EHL probe within a fluid medium, a small amount of the fluid is vaporized and super heated, turning into a rapidly expanding gas plasma bubble. The rapid expansion of this gas plasma bubble creates a hydraulic shock wave. A portion of the energy in this shock wave can impact a target calculus in close proximity to the tip of the probe, and can cause the calculus to crack into fragments.
Unfortunately, the only portion of the expanding gas bubble that is effective in transferring energy from existing probes to the target calculus is the small percentage travelling along the axis from the surface of these electrode probes to the closest portion of the target calculus. Since existing probes are generally flat across the electrode surface, whether the electrodes are parallel or coaxial, the largest portion of the energy of the expanding gas plasma bubble is dissipated radially away from this axis to the target, and does not contribute to the fragmentation of the calculus.
The inefficiency of existing EHL probes requires that clinical users employ high total energy levels to have adequate energy transfer to the target calculus. If the tips of existing probes are placed too close or in direct contact with the calculus, insufficient fluid medium may be present to allow formation of an effective gas plasma bubble and shock wave. The heat created by high voltage discharge under these circumstances only serves to heat the materials of the probe and surrounding body tissue. Within confined body structures, like the lumen of the ureter or the biliary tree, it has been well documented through clinical experience that the excessive amounts of total energy delivered via existing probes can cause damage to these delicate body tissues. Furthermore, the materials selected for construction of existing probes do not tolerate these high energy discharges creating temperatures in excess of 500.degree. F. without structural damage to these probes.
The insulation materials used to separate the electrodes on existing probes is typically of the thermoplastic type routinely used in the electronics industry as general purpose electrical insulation. These materials tend to melt and shrink due to the instantaneous high temperatures created by high voltage discharge across the probe electrodes. The electrode materials used in existing probes are typically of the types chosen for ease of handling and assembly in the electronics industry, i.e., copper, copper alloys like beryllium copper, and phosphorous bronze. These metals tend to erode easily, and to actually transfer material between electrodes due to the high voltage discharges across the probe electrodes.
These changes to the insulation separating the probe electrodes, and to the electrodes themselves, further decreases the efficiency of the lithotripsy effect obtainable from existing probes. This requires the clinical user to further increase the intensity of the total energy level delivered in an attempt to maintain usable electrohydraulic lithotripsy effect. This, in turn, increases the risk of damage to sensitive surrounding body tissue. The useful life of existing probes is often insufficient to allow completion of the desired lithotripsy treatment. The replacement of damaged electrode probes unnecessarily extends the time required for completion of the clinical procedure.
Accordingly, it is an object of the present invention to provide a device and method for use in electrohydraulic lithotripsy which achieves a more efficient fragmentation of target calculi and requires a lower total energy level.
Another object of the present invention is to provide a device and method for use in electrohydraulic lithotripsy that delivers less energy to surrounding sensitive tissue surfaces and thereby minimizes damage to such surfaces.
A still further object of the present invention is to provide a device and method for use in electrohydraulic lithotripsy that resists the damaging effects of high temperatures created by high voltage discharge across the electrodes and that resists the transfer of materials between electrodes.
A still further object of the present invention is to provide a device and method for use in electrohydraulic lithotripsy that includes a probe having an increased useful life and have a construction of smaller diameter that nevertheless is equivalent in effectiveness to known larger diameter probes.
A still further object of the present invention is to provide a device and method for use in electrohydraulic lithotripsy which does not require positioning of the probe tip in precise distance from a target calculus surface and that achieves effective EHL when in direct contact with the surface of a target calculus.